1. Field of the Invention
The present invention relates generally to the field of depth filters, and more particularly to a depth filter cartridge comprising a filter element constructed of a plurality of substantially continuous discrete filaments which are collected to form a generally tubular depth filter cartridge. The present invention also relates to a method and apparatus for making such a filter cartridge.
2. Description of the Prior Art
Several processes and apparatus for forming depth filters comprised of a plurality of substantially continuous filaments currently exist in the prior art. In this art, fiber forming devices or fiberizers are used to spray filaments of synthetic resinous material toward a rotating collection mandrel to be formed into a tubular configuration. During this process, jets of air or other gases act on the filaments to attenuate such filaments to a comparatively fine diameter and convey the same to the collection device. Several specific processes have evolved from this general concept.
One of these processes is described in U.S. Pat. No. 3,825,379 issued to Lohkamp et al. and U.S. Pat. No. 3,825,380 issued to Harding et al. Both disclose a process die or fiberizer consisting of a die head containing separate passages for the filament material and the attenuating air. During operation, molten resinous material fiber is forced through small holes in the die head toward a collection device and is attenuated by air streams positioned on opposite sides of the filament outlet holes. The collection methods utilized with this process include either a rotating drum such as shown in these patents to form a continuous mat or a rotating mandrel together with a press roller to form a tubular depth filter. This latter process is a non-continuous or semi-continuous process in which the diameters of the plurality of filaments is constant throughout the entirety of the element.
A second process is exemplified by U.S. Pat. No. 4,240,864 issued to Lin et al. This patent discloses a process die or nozzle block which delivers a plurality of filaments toward a rotating collection device. Associated with the filaments are attenuating air streams which function to attenuate the filaments as they travel toward the collection device. This patent also discloses a press roll for varying the pressure applied to the accumulating fibers on the rotating mandrel so as to provide a filter of varying fiber density. Like the process of Lohkamp et al. and Harding et al., the diameter of the individual filaments in this process is constant throughout the entirety of the filter element. However, contrary to Lohkamp et al. and Harding et al., this process is a continuous process in which the collected filaments are continuously forced off the rotating mandrel via the noncylindrical press roll to produce a coreless depth filter element.
A third specific process is represented by U.S. Pat. Nos. 4,594,202 and 4,726,901, both issued to Pall et al. Similar to the processes described above, the Pall process includes a fiberizer or fiberizer die having a plurality of individual nozzles through which the molten filament resin is forced toward a collection mandrel. Also similar to the other processes described above, this process discloses the use of air or gas streams for the purpose of attenuating the filaments as they travel toward the collection mandrel. This process differs from the processes described above, however, in that it discloses a means for varying the fiber diameter throughout the radial dimension of the filter element, while maintaining a substantially constant voids volume for each level of fiber diameter variance. Pall et al. accomplishes this by sequentially altering certain parameters which affect the fiber diameter during collection of the filaments on the rotating mandrel.
Although each of the above specific processes are generally acceptable for certain applications, each also has certain limitations. For example, one limitation of the Lohkamp et al. and Harding et al. process is that it is a non-continuous or semi-continuous process. In other words, a filter element of finite length is formed by building up a mat of attenuated filaments on a rotating mandrel. When the collected filament material reaches a desired thickness, the filter structure is removed and the process is commenced again for the next filter element. A further limitation is that the filament materials are dispensed from a common manifold. Thus, the characteristics of the collected filaments, including the filament diameters, are substantially identical throughout the entire radial thickness of the filter element. Still further, such process contemplates forming the filter structure on a separate core which remains part of the filter element when it is removed from the mandrel. If such a core is not used, a significant limitation exists in the fiber diameter which is needed to support the filter structure without collapse. This, in turn, necessarily limits the micron rating of the resulting filter, or the particle size which can be filtered.
The filter structure of Lin et al. is an improvement over the process of Lohkamp et al. and Harding et al. in that it is a continuous process for forming a continuous filter structure of indefinite length. However, Lin et al., like Lohkamp et al. and Harding et al. discloses a filter structure in which the filaments are all of the same diameter. Further, since the Lin process is designed for producing a coreless depth filter element (i.e.) a filter without a separate core, the central portion of the filter element must be formed from a filament having a diameter sufficiently large to provide support for the filter structure. This also, in turn, necessarily limits the micron rating of the filter or minimum particle size which can be filtered.
Although the Pall et al. patents contemplate a depth filter element comprised of filaments with varying diameters, there are several limitations which exist. First, the process of Pall et al. is not a continuous process, but must be repeated for each filter manufactured. Second, although some filter elements of Pall et al. have filaments of varying diameters, the process of making such elements has limitations. Specifically, the filament diameter is varied by sequentially changing one of several operating conditions of the filament producing mechanism. Whenever such a change is introduced, however, the system takes time to respond to such changes before again reaching equilibrium. The time frame for response is proportional to the degree of change. Because these changes are introduced during the manufacture of each individual filter element, a less stable and more variable process results. Further, the changeover from a filament of one diameter to that of another occurs gradually as a time related transition, rather than abruptly such as where the filaments are comprised of two or more discrete filaments.
Accordingly, although prior art methods exist for manufacturing depth filters, each of the methods, as well as the products constructed from such methods, have limitations which tend to limit the wide scale applicability of products produced by any one particular process. Accordingly, there is a need in the art for an improved, cost efficient coreless depth filter element, and more particularly, to a set of filter elements having a wide range of micron ratings and filtering applications. A need also exists for a continuous method and apparatus for producing such a filter.